<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        #divNo1{
            width: 200px;
            height: 200px;
            background-color: blue;
            margin-top: 100px;
        }
        #divNo2{
            width: 200px;
            height: 200px;
            background-color: blueviolet;
        }
        #div3{
            width: 100px;
            height: 100px;
            background-color: red;
            margin-top: 50px;
            margin-left: 50px;
            display: inline-block;
            /* 行内块元素 */
            opacity: 0.5;
            /*  颜色的透明度*/
            border-radius: 100%;
            /*  */
        }
    </style>
</head>
<body>
    <div id="divNo1">
        <div id="div3">
            No3
        </div>
    </div>
    <div id="divNo2">
        No2
    </div>
    
</body>
</html>